Can't just be a weight problem, if you can stick it in a Ti box you can put a NiCd in. More than this than meets the eye, must be a load issue.
You do know that Ti can burn under sufficiently intense conditions?
Whose going to fly on an aircraft which may or may not potentially be on fire?
The fact that a battery is locked inside a box is just part of the design change proposed.
The heat energy is not bottled in the box heating it to some gosh awful temperature, it is vented to the outside of the aircraft thus limiting the heat buildup inside the aircraft. From an engineering standpoint, complete battery runaway is a containable event. You folks fly with your engines on fire (internally) all the time, and the only time you get excited is when the fire unexpectedly goes out.
Give the engineers a little bit of credit for being able to mitigate a problem when their attention is focused on it.